Pharmacological Properties of the Type 1 Tyramine Receptor in the Diamondback Moth, Plutella xylostella.

Tyramine receptors (TARs) can be activated by tyramine (TA) or octopamine (OA) and have been shown to be related to physiological regulation (e.g., gustatory responsiveness, social organization, and learning behavior) in a range of insect species. A tyramine receptor gene in Plutella xylostella, Pxtar1, was cloned and stably expressed in the HEK-293 cell line. Pharmacological properties and expression profile of Pxtar1 were also analyzed. Tyramine could activate the PxTAR1 receptor, increasing the intracellular Ca2+ concentration ((Ca2+)i) at an EC50 of 13.1 nM and reducing forskolin (10 µM)-stimulated intracellular cAMP concentration ((cAMP)i) at an IC50 of 446 nM. DPMF (a metabolite of amitraz) and L(-)-carvone (an essential oil) were found to act as PxTAR1 receptor agonists. Conversely, yohimbine and mianserin had significant antagonistic effects on PxTAR1. In both larvae and adults, Pxtar1 had the highest expression in the head capsule and expression of Pxtar1 was higher in male than in female reproductive organs. This study reveals the temporal and spatial differences and pharmacological properties of Pxtar1 in P. xylostella and provides a strategy for screening insecticidal compounds that target PxTAR1.

Tyramine in the assessment of regional adrenergic function.

Regional adrenergic function is difficult to assess in humans. Tyramine given through a microdialysis probe may be a useful tool in this regard. However, tyramine data is hard to interpret given the drug's complex mode of action. We characterized the response to tyramine, isoproterenol, and dopamine in adipose tissue with microdialysis probes in normal subjects. We measured glycerol concentrations to follow changes in lipolysis and monitored tissue perfusion with ethanol dilution. During perfusion with tyramine, dialysate glycerol concentration increased dose-dependently from 83+/-8 microM at baseline to 181+/-18 microM at 3.5 mM tyramine (p<0.001) followed by a fall down to 121+/-9 microM at 35 mM tyramine (p<0.001). Propranolol almost completely blocked this response. A similar lipolytic response was not observed in isolated human adipocytes. Dopamine <35 microM did not replicate the tyramine-induced lipolysis; however, dopamine >35 microM potently inhibited lipolysis. We conclude that tyramine-induced lipolysis is explained by a pre-synaptic mechanism. Tyramine applied through a microdialysis probe in concentrations up to 3.5 mM can be used to assess pre- and post-synaptic mechanisms regulating lipid mobilization.

Clinical assessment of norepinephrine transporter blockade through biochemical and pharmacological profiles.

BACKGROUND: To assess the sensitivity of biochemical, physiological, and pharmacological markers of peripheral norepinephrine (NE) transporter (NET) function, we chronically antagonized NET by a range of doses of duloxetine [(+)-N-methyl-3-(1-naphthalenyloxy)-2 thiophenepropanamine], which blocks the NE reuptake process. METHODS AND RESULTS: Duloxetine was administered in a randomized, placebo-controlled study in 15 healthy volunteers. Plasma from duloxetine-treated subjects (ex vivo effect) dose-dependently decreased radioligand binding to human NET (maximum inhibition was 60%) (P=0.02). The dose of intravenous tyramine required to raise systolic blood pressure by 30 mm Hg (PD30) increased dose-dependently with duloxetine and was significant at the end of the 120-mg/d dosage (P<0.001). The plasma dihydoxyphenylglycol to NE (DHPG/NE) ratio was reduced significantly at 2 weeks of treatment with 80 mg/d duloxetine (11.3 at baseline, 3.4 at 240 mg/d, P<0.001). Plasma NE was significantly increased starting at 120 mg/d duloxetine. Urine results (corrected for 24-hour creatinine excretion) showed a dose-dependent change from the baseline urinary excretion for NE, DHPG, and the DHPG/NE ratio. The most sensitive measure, the DHPG/NE ratio, was significant at the 80-mg dose. Urinary NE excretion was significantly raised after 2 weeks of treatment with 80 mg/d duloxetine (P<0.001), the lowest dose used in the study. CONCLUSIONS: These findings suggest that the degree of NET blockade can be assessed with the plasma or urine DHPG/NE ratio and the pressor effect of tyramine. Also, the DHPG/NE ratio is more sensitive at the lower end of NET inhibition, whereas tyramine exhibits a linear relation, with NET inhibition commencing at a higher dose.

Linezolid, a novel oxazolidinone antibiotic: assessment of monoamine oxidase inhibition using pressor response to oral tyramine.

The primary objective of this study was to compare the effects of oral linezolid with moclobemide and placebo on the pressor response to oral tyramine. Secondary objectives were to determine possible mechanisms of the effect based on changes in the pharmacokinetics of tyramine and to evaluate alternative methods for quantifying the pressor effect. Subjects received linezolid (625 mg bid orally), moclobemide (150 mg tid orally), or placebo for up to 7 days. Using the oral tyramine dose producing a >30 mmHg increase in systolic blood pressure (SBP) (PD>30), a positive pressor response was defined as a PD>30 index (pretreatment/treatment ratio of PD>30) of > or = 2. There were 8/10, 11/11, and 1/10 responders with linezolid, moclobemide, and placebo, respectively. Responses returned to baseline within 2 days of drug discontinuation. The ratio of mean greatest SBP and heart rate at the time of greatest SBP (GSBP/HR) increased linearly with tyramine dose both pretreatment and during treatment with linezolid and moclobemide. During treatment, responses to tyramine when subjects took linezolid or moclobemide were significantly different from placebo. Both drugs significantly decreased tyramine oral clearance compared with placebo. Urinary excretion of catecholamines and metabolites was consistent with MAOI activity of the drugs, but results were variable. The MAOI activity of linezolid is similar to that of moclobemide, a drug used clinically without food restrictions. Restrictions to normal dietary intake of tyramine-containing foods are not warranted when taking linezolid.

6-[18F]fluorodopamine positron emission tomographic scanning in the assessment of cardiac sympathoneural function--studies in normal humans.

Thoracic positron emission tomographic (PET) scanning after injection of 6-[18F]fluorodopamine ([18F]-6F-DA) visualizes cardiac sympathetic innervation. We tested whether changes in curves relating myocardial [18F]-6F-DA-derived radioactivity with time (time-activity curves, TACs) can reflect changes in important aspects of cardiac sympathetic function. Thoracic PET scans were obtained after intravenous administration of [18F]-6F-DA or the perfusion imaging agent [13N]ammonia into normal volunteers. Ganglion blockade with trimethaphan (TRI) was used to decrease sympathoneural traffic, desipramine (DMI) to block neuronal uptake of catecholamines, and tyramine (TYR) to displace vesicular amines. After [18F]-6F-DA administration, myocardial concentrations of [18F]-6F-DA-derived radioactivity declined bi-exponentially from the peak value. TRI increased the y-intercept (yo) value for the early phase (p = 0.01), and DMI decreased the yo for the late phase (p = 0.01). The TRI effect did not result from increased arterial [18F]-6F-DA concentrations or from increased myocardial perfusion. TYR infusion, begun 90 min after [18F]-6F-DA administration, accelerated the decline of myocardial radioactivity by 2.6-fold (p = 0.003). Alterations in post-ganglionic sympathoneural traffic, neuronal catecholamine uptake, and vesicular turnover of monoamines produce distinct changes in myocardial TACs after [18F]-6F-DA injection. [18F]-6F-DA PET scanning may therefore enable assessments of effects of stressors, drugs, and neurocardiological disorders on specific aspects of cardiac sympathoneural function.

Monoamine oxidase inhibition by tranylcypromine: assessment in human volunteers.

The inhibition of monoamine oxidase (MAO) by tranylcypromine was studied in 6 healthy volunteers given increasing doses of 10, 15, 20 and 25 mg/day over a 4-week period. Measurements were made of urinary tryptamine excretion, blood pressure response to tyramine (TY) and norepinephrine (NE), and subjective self-rating. A significant increase in urinary tryptamine, indicating the onset of MAO inhibition, occurred in all 6 subjects once the cumulative dose of 40 mg TC had been administered. Thereafter, urinary tryptamine increased up to 7-fold, dose-dependently with large interindividual variation (78 +/- 27 to 549 +/- 252 microgram/g creatinine). Within 4 days after stopping the drug, control values were reached again. The assessment of TY potentiation by comparison of equieffective doses (S dose) became up to 10 times more sensitive when both the height and the duration of the increase in systolic blood pressure (S AUC) were taken into account. The increases in tyramine sensitivity found with the highest cumulative doses of TC (5.4 +/- 0.8 mg/kg; n =6) were S dose from 8-16 and S AUC from 28-162, respectively. The pharmacodynamic half-life (Pd 1/2) of TC approximated a mean first fast Pd 1/2 of 1.3 d and a slower phase of 14.2 d. During treatment with the highest TC dose, resting blood pressure was significantly elevated from 120 to 128 mm Hg, and the pressor sensitivity to NE (S NE) in 4 of the 6 subjects rose, the mean was 1.7 (n = 6). In 3 volunteers NE sensitivity was normalized within 4 days after stopping TC. There was a significant correlation between increasing vigilance with TC dose in 5 volunteers (r = 0.81, n = 15, p less than 0.01). It is concluded that combination of the results of several tests has provided reliable information about the onset, extent and duration of MAO inhibition in healthy volunteers.

An assessment of platelet aggregation induced by 5-hydroxytryptamine.

A method of expressing platelet aggregation response after incubation relative to the response before incubation has been used in preference to using direct measurements, and some reasons are given for this choice. The effect of pre-incubating human platelets with reserpine has been compared with the effect of pre-incubation with 5-hydroxytryptamine (5HT). Reserpine inhibited 5HT-induced aggregation more slowly than 5HT and on this basis their actions could be distinguished. It was found that the aggregation response of human platelets to 5HT without pre-incubation (R) is inversely proportional to the natural 5HT content of whole blood, and an explanation has been suggested.A decrease in the aggregation response of human platelets to 5HT without pre-incubation (R) with increasing age has been noted, and is commented upon with reference to the release of amines from blood platelets. Platelets in plasma obtained from control subjects and incubated with methysergide and from migrainous patients taking methysergide both have reduced aggregation responses to 5HT. Platelets incubated with clonidine (St155) do not have reduced aggregation responses. A potentiating effect was noted after 20 minutes' incubation, but platelets from migraine patients taking St155 behaved normally. Tyramine affected the response of platelets to 5HT, giving results similar to those reported for reserpine. The differences between the aggregation responses to 5HT of platelets from migraine patients and normal control subjects are discussed.

Mechanisms of reflex vasodilation: assessment of the role of neural reuptake of norepinephrine and release of histamine.

The mechanisms of reflex vasodilation were studied in an innervated canine hindlimb preparation which was perfused at a constant rate. Reflex vasodilation was produced by suddenly increasing the pressure in the trunk by the intravenous injection of norepinephrine, with consequent stimulation of the baroreceptors. When the basal vasoconstrictor tone exerted by the sympathetic nervous system on the systemic arterial bed was minimized, either by pretreatment with the alpha adrenergic blocking agent phenoxybenzamine or with reserpine, which depletes endogenous catecholamine stores, reflex vasodilation was virtually abolished. Administration of cocaine, a drug which blocks reuptake of norepinephrine by the nerve terminals, significantly reduced reflex vasodilation, the response after cocaine averaging 47% of the vasodilator response in the control period. Cocaine also potentiated the vasoconstriction caused by intra-arterially administered norepinephrine but attenuated the vasoconstriction induced by tyramine. The antihistamine, tripelennamine, had effects similar to those of cocaine. It is suggested, therefore, that reflex vasodilation results from a sudden decrease in the level of norepinephrine at the neuroeffector junction, which is a consequence of the cessation of norepinephrine secretion, together with continued and possibly augmented uptake. When the uptake mechanism is impaired, either by the administration of cocaine or tripelennamine, the magnitude of reflex vasodilation is diminished. It does not appear necessary to postulate active secretion of a vasodilator substance to account for reflex vasodilation.